Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

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607 USER COMMENTS

Karen, to place the column on the driveway you would have to cut away the area that the column will be sitting on. Then you would have to dig and pour a concrete footer to the depth of your local code requirements. Placing the column beside the driveway would be easier because you would simply dig and pour the footing and then place the column.
- Lee-Technical Service
Monday, May 2, 2016 at 10:46 AM

Susan, It would depend on how much of a slope that you need. Sakrete Top N Bond Concrete Patcher can be placed from 1/2" to a featheredge in one lift. You can add multiple lifts to achieve the desired thickness. But all of the lifts will need to be placed in the same day. Give each application an hour to set up and then add the next layer. This material can be stained following the manufacturer's recommendation for the stain application over fresh concrete. It will not work with acid stains because of the polymer in the material.
- Lee-Technical Service
Monday, May 2, 2016 at 10:26 AM

We would like to place a brick column on our driveway. Is this possible? We can't put it directly on a grass or dirt area. The concrete driveway is our only option. Thanks for your help.
- Karen
Sunday, May 1, 2016 at 8:38 AM

I am looking to add slope to a concrete porch. When it rains, the water puddles in areas. I am considering hiring someone to add a slope with perhaps this material? The concrete is in good shape. The porch is covered and it's approx. 48' x 5.5' (pretty long!) Then I want to stain the cement. Is this at all possible?
- Susan
Thursday, April 28, 2016 at 10:39 AM

Bonnie, it is possible that it did bond to the surface. If the surface of the staircase is really weak, and you resurface them with a strong material, then they can have the same effect as not bonding. You need to check the stairs and see just how bad the old material is deteriorating. Chip away some of the hollow spots and see if it has old material sticking to it. If it is, then your surface is too weak. When patching with a resurfacer, all of the loose and soft material need to be removed. Once the concrete has been taken down to a solid base you can resurface it.
- Lee-Technical Service
Tuesday, April 26, 2016 at 4:36 PM

Hi there...I have 3 cement stairs on my front porch. These are poured concrete..not a unit step. Original to the house, but deteriorating. I had them resurfaced 2 year's ago, but I see the resurfacing is just lifting off. If I tap on a repaired area..it sound's hollow, so it's obvious it did not bond with the old concrete. Any suggestion's??? I don't want to have to jackhammer them. Any help you can give me would be greatly appreciated. Thanks, Bonnie
- bonnie kohrs
Sunday, April 24, 2016 at 1:13 PM

Tom, If the surface is properly prepped then you would have a lasting repair. All of the loose and soft material must be removed prior to resurfacing the driveway. The surface will also need to be damp prior to application. The key to a lasting repair is your prep work. Also check your forecast to avoid rain.
- Lee-Technical Service
Thursday, April 21, 2016 at 8:30 AM

I had a section of driveway replaced in December. Shortly after it rained and of course the concrete was not cured so there was a good deal of concrete washing away as well as spalling. A few days latter the contractor returned and skim coated the section - and again rain and most of the skim coat scaled away. He now wants to reskim the section - temps are now in 80's. Will this be a lasting repair or will this fail in a short period of time?
- Tom
Tuesday, April 19, 2016 at 8:41 PM

Rob Smith, No, we do not recommend this type of application for Top N Bond.
- Chris Technical Services
Monday, April 18, 2016 at 2:11 PM

Beth, unfortunately we do not manufacture a glue for your application. We recommend contacting a glue/adhesive manufacture for their recommendations.
- Chris Technical Services
Monday, April 18, 2016 at 2:09 PM

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